Study on Model Response of ACFM for Rail RCF Crack Based on Destructive Field Test

The detection of rail rolling contact fatigue (RCF) crack by alternative current field measurement (ACFM) is hindered by the unclear parameters and the relationship between ACFM response and crack parameters. In this article, the response features of ACFM to rail RCF crack parameters are analyzed based on the destructive field test from new rail to 320 million gross ton (MGT) decommission and the established air-crack-rail ACFM model. The morphology and parameters of RCF crack are obtained from CT scan, surface observation, microscope observation, and recalculation of rail blocks sampled with passing gross load of 10, 30, 62, 100, 150, 210, 275, and 320 MGT. The response of ACFM is calculated and extracted at the most significant peak and trough of Bx and Bz. The results show that the crack parameters, surface length (SL), ellipse ratio (ER), and internal angle (IA) of rail RCF crack in eight sampled rail blocks, are 17.2–25 mm, 1.8–6, and 9.1°–38.3°, respectively. The significant response features are comprehensively influenced by all crack parameters. With the change of crack parameters, a clear rule can be found in the response of ACFM features. For the peak of Bx and Bz, the response is dominated by SL followed by ER and IA, and the maximum influences of SL are 85.4% and 77.5%. For the trough of Bx, the responses to internal parameters ER and IA are in a greater proportion. The maximum influences of ER and IA to trough of Bx are 58.1% and 53.8%, respectively.